The present invention relates to a dispersion compensating optical fiber that is connected to, for example, a single-mode optical fiber having zero dispersion in the vicinity of a wavelength of 1.3 xcexcm (hereinafter, referred to simply as single-mode optical fiber) and is used in making wavelength division multiplexing optical transmission by the use of light in a 1.5 xcexcm wavelength band or other predetermined wavelength bands, and the invention relates to an optical transmission line that uses the dispersion compensating optical fiber.
Single-mode optical fibers have been constructed as a transmission network of optical communications all over the world. Nowadays, the amount of communications information is showing a tendency to increase sharply with the development of the information society. With the increase in information, wavelength division multiplexing transmission (WDM transmission) has been widely accepted in the field of telecommunications such that the era of the WDM transmission has now been entered. The WDM transmission is an optical transmission method suitable for large capacity high-speed communication because the WDM transmission can transmit light having a plurality of wavelengths by means of one optical fiber.
However, when the WDM transmission is made by the use of existing single-mode optical fibers and by the use of a signal light beam in a 1.3 xcexcm-wavelength band, it does not coincide, in a wave range, with a 1.55 xcexcm-wavelength band that is the gain band of a general optical amplifier using an erbium doped optical fiber. (Note that the 1.55 xcexcm-wavelength band is a wavelength band substantially centering around a wavelength of 1550 nm, including wavelengths of, for example, 1530 nm to 1570nm; the term xe2x80x9c1.55 xcexcm-wavelength bandxe2x80x9d is hereinafter used with this meaning.) Thus, a problem resides in that the optical amplifier cannot be used, and an obstacle is caused to long distance optical communication (long distance optical transmission). Therefore, recently, WDM transmission has been made in the 1.55 xcexcm-wavelength band using the existing single-mode optical fibers.
However, when optical communication is carried out in the 1.55 xcexcm-wavelength band by using the existing single-mode optical fiber, the single-mode optical fiber usually has a positive dispersion of about 17 ps/nm/km in a wavelength of 1.55 xcexcm that is substantially a center wavelength in the 1.55 xcexcm-wavelength band, and, in addition, has a positive dispersion slope in the 1.55 xcexcm-wavelength band. For this reason, a disadvantageous case occurred in which the waveform distortion of an optical signal becomes large as the optical signal propagates through the single-mode optical fiber, and, especially when the WDM optical transmission is made, it becomes difficult to separate/distinguish the signal on the signal receiving side, thus lowering the quality of the optical communication and destroying the reliability thereof.
Then, in order to resolve the problem, recently, a dispersion compensating optical fiber that has large negative dispersion in the 1.55 xcexcm-wavelength band has been modularized and has been connected to the single-mode optical fiber transmission line, in order to compensate for the dispersion and the dispersion slope in the 1.55 xcexcm-wavelength band by a short dispersion compensating optical fiber. This aims to enable the WDM transmission in the 1.55 xcexcm-wavelength band by compensating the positive dispersion and the positive dispersion slope that the single-mode optical fiber has in the 1.55 xcexcm-wavelength band so as to restrain the deterioration of a transmission signal caused by wavelength dispersion.
However, as described above, the conventional dispersion compensating optical fiber is designed to compensate for the dispersion and dispersion slope of the single-mode optical fiber by its short length, and therefore, generally, the mode field diameter in the 1.55 xcexcm-wavelength band becomes small in design so as to easily cause a nonlinear phenomenon. Additionally, a problem resides in that a transmission loss in the 1.55 xcexcm-wavelength band is enlarged to be 0.4 dB/km or more, and a polarization mode dispersion value (PMD value) in the 1.55 xcexcm-wavelength band also reaches a relatively large value of 0.2 ps/kmxc2xd or more. The single-mode optical fiber is relatively excellent in low nonlinearity.
Today, there is a demand to increase the amount of communications information even more. However, if the WDM transmission uses only the 1.55 xcexcm-wavelength band, there is a limit to the number of transmissible wavelengths, and a saturation state will eventually be reached. Therefore, there are demands for new optical transmission lines in which a 1.5 xcexcm-wavelength band (i.e., a wavelength band of, for example, 1520 through 1620 nm, including the conventional 1.55 xcexcm-wavelength band; the term xe2x80x9c1.5 xcexcm-wavelength bandxe2x80x9d is hereinafter used with this meaning) is made usable by extending a usable wavelength band used for the wavelength division multiplexing transmission to both sides of the conventional 1.55 xcexcm-wavelength band (1530 through 1570 nm, for example), or a wavelength band (wavelength range) other than the 1.5 xcexcm-wavelength band is also made usable.
However, if the conventional dispersion compensating optical fiber is connected to the single-mode optical fiber, the dispersion of, for example, wavelengths 1530 through 1570 nm in the 1.55 xcexcm-wavelength band is compensated to be almost zero, but, instead, the dispersion in other wavelength ranges including a 1.3 xcexcm-wavelength band that has shown almost zero dispersion before the connection to the conventional dispersion compensating optical fiber is displaced from the neighborhood of zero. The optical transmission line formed by connecting the conventional dispersion compensating optical fiber to the single-mode optical fiber in this way cannot serve as an optical transmission line capable of making the WDM transmission using the other wavelength ranges that include the 1.3 xcexcm-wavelength band, for example.
Then, the present inventor has reoriented the conventional concept that the dispersion compensation of the single-mode optical fiber is merely carried out with the short dispersion compensating optical fiber, and has studied ways to realize an optical fiber provided with a function suitable for long distance optical transmission of the dispersion compensating optical fiber in such a way as to improve the characteristics of the mode field diameter, bend loss, and polarization mode dispersion of the dispersion compensating optical fiber itself in the 1.5 xcexcm-wavelength band.
If such an optical fiber is realized, a problem caused by the nonlinearity can be controlled, and excellent signal transmission in the 1.5 xcexcm-wavelength band can be made in the optical transmission line in which the single-mode optical fiber and the dispersion compensating optical fiber are connected to each other.
Additionally, the present inventor thought that an optical transmission line having small dispersion in a wavelength band, for example, near 1.3 xcexcm that is a zero dispersion wavelength of the single-mode optical fiber will be formable if the dispersion characteristics of the dispersion compensating optical fiber to be connected to the single-mode optical fiber are made appropriate.
The present invention was made in consideration of the foregoing. A first object of the present invention is to provided a dispersion compensating optical fiber in which the characteristics of a mode field diameter, bend loss, polarization mode dispersion, etc., in the 1.5 xcexcm-wavelength band are excellent, and, by connecting to a single-mode optical fiber or connecting to an optical fiber having almost the same dispersion characteristic in the 1.5 xcexcm-wavelength band as the single-mode optical fiber, an optical signal can be transmitted for a long distance while compensating the dispersion in the 1.5 xcexcm-wavelength band of light travelling through the optical fiber, and the dispersion can be reduced also in wavelengths other than the 1.5 xcexcm wavelength band. A second object of the present invention is to provide an optical transmission line capable of, by using the aforementioned dispersion compensating optical fibers, making high-quality WDM transmission in which waveform distortion is controlled.
In order to achieve the objects, the present invention provides dispersion compensating optical fibers constructed as follows. That is, there is provided a dispersion compensating optical fiber used by connection to a single-mode optical fiber that has zero dispersion in the vicinity of a wavelength of 1.3 xcexcm or by connection to a compensated optical fiber whose dispersion characteristic in a 1.5 xcexcm-wavelength band is substantially the same as that of the single-mode optical fiber, the dispersion compensating optical fiber characterized in that a dispersion value in a predetermined wavelength band in the 1.5 xcexcm-wavelength band of a connected optical fiber is set at xe2x88x921 ps/nm/km through 1 ps/nm/km, and a dispersion value in one or more predetermined wavelength bands different from the predetermined wavelength band in the 1.5 xcexcm-wavelength band is set at xe2x88x925 ps/nm/km through 5 ps/nm/km.
As a feature of the present invention, the predetermined wavelength band different from the 1.5 xcexcm-wavelength band in which the dispersion value of the connected optical fiber is set at xe2x88x925 ps/nm/km through 5 ps/nm/km is a 1.3 xcexcm-wavelength band.
According to another aspect of the present invention, there is provided a dispersion compensating optical fiber characterized in that a mode field diameter in a predetermined wavelength band in a 1.5 xcexcm-wavelength band or a mode field diameter in a wavelength in the vicinity of a center of the predetermined wavelength band is 6.3 xcexcm or more, and, by connection to a single-mode optical fiber that has zero dispersion in the vicinity of a wavelength of 1.3 xcexcm or by connection to a compensated optical fiber whose dispersion characteristic in the 1.5 xcexcm-wavelength band is substantially the same as that of the single-mode optical fiber, a dispersion value in the predetermined wavelength band in the 1.5 xcexcm-wavelength band of a connected optical fiber is set at xe2x88x921 ps/nm/km through 1 ps/nm/km.
According to still another aspect of the present invention, there is provided a dispersion compensating optical fiber characterized in that a mode field diameter in a predetermined wavelength band in a 1.5 xcexcm-wavelength band or a mode field diameter in a wavelength in the vicinity of a center of the predetermined wavelength band is 5.5 xcexcm or more, and a bend loss in a bend diameter of 20 mm in the predetermined wavelength band or in the wavelength in the vicinity of the center of the predetermined wavelength band is 3.0 dB/m or less, and, by connection to a single-mode optical fiber that has zero dispersion in the vicinity of a 1.3 xcexcm wavelength or by connection to a compensated optical fiber whose dispersion characteristic in the 1.5 xcexcm-wavelength band is substantially the same as that of the single-mode optical fiber, a dispersion value in the predetermined wavelength band in the 1.5 xcexcm-wavelength band or in the vicinity of the center of the predetermined wavelength band of a connected optical fiber is set at xe2x88x921 ps/nm/km through 1 ps/nm/km.
In the dispersion compensating optical fibers mentioned above, a dispersion value in the predetermined wavelength band in the 1.5 xcexcm-wavelength band or a dispersion value in the wavelength in the vicinity of the center of the predetermined wavelength band is xe2x88x9220 ps/nm/km through 10 ps/nm/km, and a transmission loss in the predetermined wavelength band or in the wavelength in the vicinity of the center of the predetermined wavelength band is 0.25 dB/km or less, and, by connection to the single-mode optical fiber that has zero dispersion in the vicinity of a 1.3 xcexcm wavelength or by connection to the compensated optical fiber whose dispersion characteristic in the 1.5 xcexcm-wavelength band is substantially the same as that of the single-mode optical fiber, a dispersion value in the predetermined wavelength band in the 1.5 xcexcm-wavelength band or in the vicinity of the center of the predetermined wavelength band of the connected optical fiber is set at xe2x88x921 ps/nm/km through 1 ps/nm/km.
Preferably, a dispersion slope either in the predetermined wavelength in the 1.5 xcexcm-wavelength band or in the 1.3 xcexcm-wavelength band or a dispersion slope both in the predetermined wavelength in the 1.5 xcexcm-wavelength band and in the 1.3 xcexcm-wavelength band is negative.
In an aspect of the present invention, a transmission loss in the predetermined wavelength band in the 1.5 xcexcm-wavelength band or in the wavelength in the vicinity of the center of the predetermined wavelength band is set to be 0.30 dB/km or less, and a polarization mode dispersion value in the predetermined wavelength band or in the wavelength in the vicinity of the center of the predetermined wavelength band is set to be 0.15 ps/kmxc2xd or less, and a bend loss in a bend diameter of 20 mm in the predetermined wavelength band or in the wavelength in the vicinity of the center of the predetermined wavelength band is set to be 20 dB/m or less.
Furthermore, the dispersion compensating optical fiber of the present invention is characterized in that a transmission loss in the predetermined wavelength band in the 1.5 xcexcm-wavelength band or in the wavelength in the vicinity of the center of the predetermined wavelength band is set to be 0.30 dB/km or less, and a polarization mode dispersion value in the predetermined wavelength band or in the wavelength in the vicinity of the center of the predetermined wavelength band is set to be 0.15 ps/kmxc2xd or less, and a bend loss in a bend diameter of 20 mm in the predetermined wavelength band or in the wavelength in the vicinity of the center of the predetermined wavelength band is set to be 20 dB/m or less.
In another aspect of the present invention, the dispersion compensating optical fiber is characterized in that the dispersion compensating optical fiber is formed by covering an outer side of a center core with a first side core, covering an outer side of the first side core with a second side core, and covering an outer side of the second side core with a cladding, and the relations xcex941 greater than xcex943 greater than xcex942, and 0.8%xe2x89xa6xcex941xe2x89xa61.3%, xe2x88x920.4%xe2x89xa6xcex942xe2x89xa6xe2x88x920.2% (or xe2x88x920.5xe2x89xa6xcex942/xcex941xe2x89xa6xe2x88x920.35), and 0.2%xe2x89xa6xcex943xe2x89xa60.3% are established wherein xcex941 is a comparative refractive index difference of the center core to the cladding, xcex942 is a comparative refractive index difference of the first side core to the cladding, and xcex943 is a comparative refractive index difference of the second side core to the cladding, and an outer diameter of the first side core is 2 through 2.5 times as large as an outer diameter of the center core, and an outer diameter of the second side core is 2.5 through 3.5 times as large as the outer diameter of the center core.
Furthermore, according to the present invention, there is provided an optical transmission line whose dispersion value in a predetermined wavelength band in the 1.5 xcexcm-wavelength band is xe2x88x921 ps/nm/km through 1 ps/nm/km by connecting any one of the aforementioned dispersion compensating optical fibers to the single-mode optical fiber that has zero dispersion in the vicinity of a wavelength of 1.3 xcexcm or to the compensated optical fiber whose dispersion characteristic in the 1.5 xcexcm-wavelength band is substantially the same as that of the single-mode optical fiber.
In the optical transmission line according to an aspect of the present invention, a dispersion value in one or more predetermined wavelength bands different from the 1.5 xcexcm-wavelength band is xe2x88x925 ps/nm/km through 5 ps/nm/km, and a dispersion value in the 1.3 xcexcm-wavelength band is xe2x88x925 ps/nm/km through 5 ps/nm/km.
Preferably, the optical transmission line is connected by a fusion splice to the single-mode optical fiber that has zero dispersion in the vicinity of a wavelength of 1.3 xcexcm or to the compensated optical fiber whose dispersion characteristic in the 1.5 xcexcm-wavelength band is substantially the same as that of the single-mode optical fiber, and loss at a fusion splice 0.4 dB or less.
In another preferable form of the optical transmission line of the present invention, the dispersion compensating optical fiber is connected to the single-mode optical fiber that has zero dispersion in the vicinity of a wavelength of 1.3 xcexcm or to the compensated optical fiber whose dispersion characteristic in the 1.5 xcexcm-wavelength band is substantially the same as that of the single-mode optical fiber, with a dispersion shift optical fiber that has zero dispersion in the 1.5 xcexcm-wavelength band interposed between the dispersion compensating optical fiber and the single-mode optical fiber or the compensated optical fiber, and a length of the dispersion shift optical fiber is 1/1000 times or less as long as the single-mode optical fiber or the compensated optical fiber connected to the dispersion compensating optical fiber, and a mode field diameter in the 1.5 xcexcm-wavelength band of the dispersion shift optical fiber is larger than that of the dispersion compensating optical fiber and less than that of single-mode optical fiber or that of the compensated optical fiber.
By connecting the dispersion compensating optical fiber of the present invention as described above to the single-mode optical fiber or to the compensated optical fiber whose dispersion characteristic in the 1.5 xcexcm-wavelength band is substantially the same as that of the single-mode optical fiber, the dispersion value in the predetermined wavelength band in the 1.5 xcexcm-wavelength band is set at xe2x88x921 ps/nm/km through 1 ps/nm/km. Accordingly, the dispersion in this wavelength band is compensated to be almost zero, and the dispersion value in one or more predetermined wavelength bands (the 1.3 xcexcm-wavelength band, for example) different from the 1.5 xcexcm-wavelength band becomes xe2x88x925 ps/nm/km through 5 ps/nm/km, so that the dispersion value in the predetermined wavelength band becomes small to such an extent as not to exert a bad influence on wavelength division multiplexing transmission.
Therefore, by connecting the dispersion compensating optical fiber of the present invention to the single-mode optical fiber that has zero dispersion in the vicinity of a wavelength of 1.3 xcexcm or to the compensated optical fiber whose dispersion characteristic in the 1.5 xcexcm-wavelength band is substantially the same as that of the single-mode optical fiber, it becomes possible, not only in the predetermined wavelength band in the 1.5 xcexcm-wavelength band but also in the other predetermined wavelength bands (1.3 xcexcm-wavelength band, for example) different from the 1.5 xcexcm-wavelength band, to make WDM transmission in which the distortion of a signal light beam caused by the distortion is small and is high in quality. This makes it possible to widen the wavelength range of the WDM transmission.
Additionally, the dispersion compensating optical fiber of the present invention is superior in terms of the characteristic of WDM transmission in a predetermined wavelength band in the 1.5 xcexcm-wavelength band or in a wavelength in the vicinity of the center of the predetermined wavelength band in the 1.5 xcexcm-wavelength band, and, by connecting the dispersion compensating optical fiber to the single-mode optical fiber that has zero dispersion in the vicinity of a wavelength of 1.3 xcexcm or to the compensated optical fiber whose dispersion characteristic in the 1.5 xcexcm-wavelength band is substantially the same as that of the single-mode optical fiber, the dispersion value in the predetermined wavelength band in the 1.5 xcexcm-wavelength band of the connected optical fiber can be set to be xe2x88x921 ps/nm/km through 1 ps/nm/km. Accordingly, it is possible to compensate for the dispersion of the single-mode optical fiber in the predetermined wavelength band in the 1.5 xcexcm-wavelength band and that of the compensated optical fiber therein to be almost zero.
Therefore, by connecting the dispersion compensating optical fiber to the single-mode optical fiber or to the compensated optical fiber, high-quality long-distance transmission can be made in the predetermined wavelength band in the 1.5 xcexcm-wavelength band while compensating the dispersion in this wavelength band of the single-mode optical fiber or the compensated optical fiber.
Furthermore, by connecting the dispersion compensating optical fiber of the present invention in which a transmission loss in a predetermined wavelength band in the 1.5 xcexcm-wavelength band or in a wavelength in the vicinity of the center of the predetermined wavelength band in the 1.5 xcexcm-wavelength band shows a small value of 0.25 dB/km or less either to the single-mode optical fiber or to the compensated optical fiber, high-quality long-distance transmission can be made in the predetermined wavelength band in the 1.5 xcexcm-wavelength band while compensating the dispersion in this wavelength band of the single-mode optical fiber or the compensated optical fiber.
Furthermore, according to the dispersion compensating optical fiber of the present invention whose dispersion slope in the 1.5 xcexcm-wavelength band is negative, it is possible to flatten the dispersion in the predetermined wavelength band in the 1.5 xcexcm-wavelength band of the connected optical fiber, while compensating the positive dispersion slope in the predetermined wavelength band in the 1.5 xcexcm-wavelength band of the single-mode optical fiber or the compensated optical fiber, by connecting the dispersion compensating optical fiber to the single-mode optical fiber or to the compensated optical fiber whose dispersion characteristic in the 1.5 xcexcm-wavelength band is substantially the same as that of the single-mode optical fiber.
Furthermore, according to the dispersion compensating optical fiber of the present invention whose dispersion slope in the 1.3 xcexcm-wavelength band is negative, it is possible to flatten the dispersion in the 1.3 xcexcm-wavelength band of the connected optical fiber, while compensating the positive dispersion slope in the 1.3 xcexcm-wavelength band of the single-mode optical fiber or the compensated optical fiber, by connecting the dispersion compensating optical fiber to the single-mode optical fiber or to the compensated optical fiber whose dispersion characteristic in the 1.5 xcexcm-wavelength band is substantially the same as that of the single-mode optical fiber.
Furthermore, according to the dispersion compensating optical fiber of the present invention in which a transmission loss in a predetermined wavelength band in the 1.5 xcexcm-wavelength band or in a wavelength in the vicinity of the center of the predetermined wavelength band is 0.30 dB/km or less, a polarization mode dispersion value in the predetermined wavelength band or in the wavelength in the vicinity of the center of the predetermined wavelength band is 0.15 ps/kmxc2xd or less, and a bend loss in a bend diameter of 20 mm in the predetermined wavelength band or in the wavelength in the vicinity of the center of the predetermined wavelength band is 20 dB/m or less, the quality of WDM transmission in the predetermined wavelength band in the 1.5 xcexcm-wavelength band can be improved, in addition to any advantageous effect of a first through tenth inventions mentioned above, because of the superiority of the WDM transmission characteristic in the predetermined wavelength band (1.55 xcexcm-wavelength band, for example) in the 1.5 xcexcm-wavelength band or in the wavelength in the vicinity of the center of the predetermined wavelength band.
Furthermore, according to the dispersion compensating optical fiber of the present invention in which the relations xcex941 greater than xcex943 greater than xcex942, 0.8%xe2x89xa6xcex941xe2x89xa61.3% xe2x88x920.4xe2x89xa6xcex942xe2x89xa6xe2x88x920.2% (or xe2x88x920.5xe2x89xa6xcex942/xcex941xe2x89xa6xe2x88x920.35), and 0.2%xe2x89xa6xcex943xe2x89xa60.3% are established, the refractive index profile of the dispersion compensating optical fiber that has the advantageous effect can be optimized.
Furthermore, since the optical transmission line of the present invention is formed by connecting the dispersion compensating optical fiber to the single-mode optical fiber or to the compensated optical fiber whose dispersion characteristic in the 1.5 xcexcm-wavelength band is substantially the same as that of the single-mode optical fiber, the optical transmission line can restrict the waveform distortion caused by the dispersion in the predetermined wavelength band in the 1.5 xcexcm-wavelength band while keeping the dispersion value in this wavelength band at xe2x88x921 ps/nm/km through 1 ps/nm/km, and can make high-quality WDM transmission.
Especially, according to the optical transmission line of the present invention whose dispersion value in one or more predetermined wavelength bands different from the 1.5 xcexcm-wavelength band is xe2x88x925 ps/nm/km through 5 ps/nm/km, the waveform distortion caused by the dispersion in this predetermined wavelength band is restricted, thereby WDM transmission can be made, and the communication wavelength range can be widened.
Likewise, according to the optical transmission line of the present invention whose dispersion value in the 1.3 xcexcm-wavelength band is xe2x88x925 ps/nm/km through 5 ps/nm/km, the waveform distortion caused by the dispersion in the 1.3 xcexcm-wavelength band is restricted, thereby WDM transmission can be made, and the communication wavelength range can be widened.
Furthermore, according to the optical transmission line of the present invention whose fusion splice loss between the single-mode optical fiber or the compensated optical fiber and the dispersion compensating optical fiber is 0.4 dB or less, the quality of the optical transmission is prevented from decreasing because of the fusion splice loss, and therefore WDM transmission of an even higher quality can be achieved.
Furthermore, according to the optical transmission line of the present invention connected through the medium of the dispersion shift optical fiber that has zero dispersion in the 1.5 xcexcm-wavelength band, for the connection between the single-mode optical fiber or the compensated optical fiber and the dispersion compensating optical fiber, the dispersion shift optical fiber whose length is short and whose mode field diameter is intermediate between the mode field diameters of the aforementioned fibers is interposed therebetween, and therefore it is possible to reduce the connection loss of the optical fibers and make high-quality WDM transmission even if the mode field diameter of the dispersion compensating optical fiber is small.